The present invention relates to a method and apparatus for the conversion of crystalline silica into amorphous silica.
Amorphous silica is widely used to form, usually by slip-casting methods, various refractory bodies and articles for high temperature applications in the chemical and metalurgical industries. Because the coefficient of linear expansion of cristobalite differs significantly from that of amorphous silica, the presence of significant amounts of cristobalite in the finished refractory products will render those products unsuitable for use in high temperature applications. Significant amounts of cristobalite within the articles will cause them to crack apart when subjected to high temperatures.
It is well known in the art that any and all crystalline forms of silica are converted into the amorphous state when heated above 1728.degree. C., the melting point of cristobalite. The prior art has commonly employed electrical arc furnaces to produce the thermal energy required for the fusion and conversion of the crystalline silica. However, the electrical arc furnace is known to produce temperatures much higher than those required for fusion of silica and therefore consumes more energy than required for conversion. In addition to the thermal losses associated with the electric arc processes, super-heating of the silica charge results in a high rate of consumption of the carbon or graphite electrodes and a loss of significant amounts of silica through vaporization. Another drawback of the prior art arc melting process resides in the production of gaseous by-products which are formed within the furnace as a function of the temperature and degree of super-heat of the carbon electrodes and silica charge. The unwanted by-product reactions include those resulting in the formation of silicon carbide and the very toxic gas carbon monoxide. The prior art apparatus and process allow varying amounts of carbon monoxide to escape from the furnace, thus presenting a safety hazard and an environmental problem. Silicon carbide also forms in a gaseous phase reaction but tends to crystallize on the carbon or graphite electrodes, if the electrodes are freely suspended above the melt.
Still another drawback of the prior art technology results from direct contact between the hot fused silica and the heated carbon or graphite electrodes. Such direct contact results in the formation of silicon carbide which tends to disassociate at the surface of the electrodes resulting in the formation of silicon vapor and carbon monoxide gas. The direct contact between the hot fused silica and the electrodes also results in contamination of the fused silica product by carbon, silicon carbide, or even silicon. The contamination of the fused silica tends to turn its color from white into gray, brown or black. Since the contaminated silica is not acceptable for many end use applications, a physical separation of the contaminated products from the uncontaminated is necessary.
The prior art apparatus and process employing silica sand as a refractory material, surrounding the melt, encounter further difficulties. The prior art technique for insulating the melt results in the adherence of unfused silica sand and layers of cristobalite to the converted product. The skin must be removed prior to any further processing since it represents an unwanted contaminating material. Furthermore, the thermal conductivity, particularly the radiation conductivity of fused silica and crystalline silica, is comparatively high at temperatures near the melting point of cristobalite, leading to significant losses of thermal energy.
In devices wherein the resistance elements run through the furnace, with terminal ends located at each end of the furance housing, considerable thermal losses were found to occur because of the inevitable flow of heat from the resistance heating element or electrode into the connectors. In addition to such thermal losses, losses of electrical power were encountered as a result of the increase of the reactance of the furnace circuit with the introduction of an appreciable self-inductance as a result of looping the power feed lines.
Rotatable resistance furnaces, such as that disclosed in U.S. Pat. No. 2,936,505 issued to Witucki et al, have been employed in other applications. However, such apparatus is unsuitable for the production of amorphous silica because it does not provide for rapid discharge and quenching of the hot product, which rapid discharging is necessary to minimize cristobalite formation during cooling of the product.